Method of forming coil insulation



Jan. 6, 1942. DAWSON, JR 2,269,185

METHOD OF FORMING COIL INSULATION Filed Nov. 15, 1939 Inventor: Edwarfd 5. Dawson J71,

by His Attorney Patented Jan. 6, 1942 UNITED STATES PATENT OFFICE 2,269,185 v METHOD OF FORMING COIL INSULATION Edward S. Dawson, Jr., Schenectady, N. Y., assignor to General Electric Company; a corporation of New York Application November 15, 1939, Serial No. 394,513

(oi. 17s 21-) 2 Claims.

' insulation. These materials, no matter how thoroughly dried, contain some moisture rendering them more susceptible to electrical break down during use. To render the fibrous insulation more water repellant, such impregnants as asphalts or varnishes-are used. The coil wrapped with such impregnated fibrous materials, however, is bulky and the insulating value of the impregnated fibrous material is low as compared with its thickness.

To overcome these disadvantages, it has been proposed to use tapes formed of cellulose acetate or other cellulose ester as coil insulation and various methods of applying this type of insulation have been suggested. For example, U. S. Patent 1,874,723 to Edward S. Dawson, discloses an insulated coil completely insulated with a cellulose ester tape sealed and shrunk onto the coil by heat treatment. It is therein suggested that the heat should be applied by passing a current through the conductors, by molding the coil structure in a steam heated mold, or by subjecting the coil to the action of a high frequency field for a few seconds. Although for certain purposes, the methods set out in this patent have proven satisfactory, their use has not always resulted in a cell having the desired maximum electrical properties. It has been found that these heating methods, difiicult to control, may cause excessivebubbling of the cellulosic material resulting in undesirable voids in the insulation and causing assembly diiiiculties. Further, the comparatively thick cellulose ester tape used in practicing the patented method contains some volatile ingredients after the heat treatment is completed, which ingredients are harmful to the electrical properties of the coil as'a whole.

It is, therefore, an object of my invention to provide a new and improved method of forming an insulated electrical coil, the major coil insulation of which has outstanding electrical properties and is free from voids and is sealed against oil, dirt, grease and moisture.

Another object of my invention is to provide a coil with a major insulation free from ingredients which may volatilize at operating temperatures and which tend to lower the electrical value of the insulation.

A further object is to provide a positive method for producing an insulated coil which will not puff or deteriorate under heat and electrical stresses. Further objects of my invention will become apparent to those skilled in the art from the following description of my invention taken in con-= nection with the accompanying drawing in which Fig. l is a view in perspective of a portion of an induction motor field coil 5, with part of the outer layers of insulation removed, which has been insulated in accordance with my invention; and Fig. 2 shows a combination cellulose ester-mica tape used as part or all of the major coil insulation.

in carrying out my invention, suitable conducting bars or wires are first individually insulated in the usual manner with a number of layersor wrappings 2 of cellulose acetate, glassor other known insulating material, such as cotton, with or without the usual impregnants. The completely insulated conductors are formed then into coils of the proper shape as, for example, in the form of 'an induction motor field coil l, as shown in Fig 1.

The formed coil is next covered with a number of wrappings of the combination cellulose estermica tape 3, shown in detail in Fig. 2, and which suitablymay comprise a layer of mica flakes or particles 4 sandwiched between two protective, or backing, sheets 5 of very thin plasticized cellulose ester or other cellulose derivative. Although I prefer to use a tape in which the mica layer is interposed between two sheets of cellulose derivative, it is within the scope of my invention that one of the sheets be omitted. The tape resulting from this omission is best wrapped onto the coil with the mica facing on the inside of the wrap so that the shrinkage of the cellulose acetate will closely bind the mica layer. V

The combination tape 3 may be prepared by standard methods. For example, the mica particles may be dusted or sifted onto one face of a cellulose ester sheet plasticized with enough material, such as tricresyl phosphate or dibutyl phthalate, to insure flexibility of the final tape, which sheet previously has been coated with anysuitable adhesive, such as a solution of a cellulose derivative or of a vinyl compound. If a second cellulose derivative layer is to be applied on top of the mica layer, the surface of this second sheet bearing on the mica layer should also be coated with an adhesive material. Obviously, additional mica and cellulose derivative layers may be added although this can be done only at a sacrifice of the flexibility of the combination tape. After the cellulose-mica layers have been brought into proper relationship, the whole may be passed through heated rolls to unite firmly the various layers.

A number of layers or wrappings of this combination tape 3 are then applied to the entire coil l until the desired insulation thickness has been obtained. Because of the high plasticizer or softener content of the cellulose ester layers, the tape can be closely wrapped onto the coilwithout any tendency to bag or bulge even tlfough such coils are of rectangular or irregular cross-section. Sacrifice tape, such as ordinary linen tape is then applied to the coil to i hold the combination tape implace and to protect it during the subsequent operations and the coil structure is introduced into a vacuum-tight ester-mica tape followed by several dips of var-. nish to protect the celluloseester-mica tape dur-" ing assembly of the motor or other electrical device in which the coil is to be used.

It is desirable that the cellulose ester sheets be as thinas possible in order that the proportion of the high dielectric constant mica will be high, Because thin sheets of the cellulose ester are used, ordinary methods of heat treating the chamber where it is subjected to a combined vachum and heat treatment for such a time and at such a temperature as to remove all traces of the plasticizer and other volatile ingredients from the cellulose derivative layers and other portions coil are often so drastic as to destroy completely the cellulose ester backing without accomplish-.

' ing the desired shrinking and binding action. I prefer to use thin cellulose sheets having a a 0.8 mil. r By my above-described process, I am able to of the coil structure. The primary objectof this vacuum treatment is to remove the volatile plasticizers and solvents present in the cellufree from bubbles or other irregularities.

lose-derivative, which volatiles have been found 7 provided with a high dielectric insulation, sealed to seriously impair certain desirable properties,

such as the dielectric constant and heat resis- 3 tivity of the tape; As an example of this heat and vacuum treatment, it may be stated that for ordinary coils, a cycle of four hours heating at 160 C. followed by five or six hours vacuum treatment at the same temperature will suffice to remove the volatile materials.

Following the vacuum treatment, the coil is subjected to further heating under pressuraprefpresent in the underlying coiled conductor structure. Only by following this method have I been able to eliminate completely all of the voids wit their accompanying detrimental effect Im the electrical properties of the insulation. To accom- Dlish this, a suitable highly viscous liquid, such 1 as an oil-modified asphalt which. has a relativei like in the molding 011 the major 0011 insulation ly high'viscosity at the coil treating temperature and which is heated to approximately the same temperature used in the vacuum treatment, is

introduced into the chamber or tank and a presi sure of the order of 100 pounds per square inch is applied to the fluid compound for about three or four hours. v

By this treatment involving the consecutive use of heat, vacuum plus heat, and pressure plus heat, in treating the coil and.the major insulation thereon, what little solvent Temains in the mica tape adhesiveis removed and the cellulose ester, due to the heat and due to the removal of thickness of less'than 1 mil and obtain a coil the surface'of which is smooth and Because of the materials ,used; the finished coil is contains none of the usual low melting point im-,

pregnants which tend to collect at certain points during use of the coil in a motor or the like at the expense of the insulating value of the remaining portions of the coil. A coil insulated in accordance with my improved method has been subjected to 100 C. and electrically stressed over 700 hours without the slightest evidence of puffiriig or deterioration under active corona condit ons.

A further advantage of my process obtained from the fluid pressure step is that the coil loops are as adequately and uniformly compressed .and sealed as are the slot portions of the coil. This is not possible with ordinary methods involving the use of heated steel molds and the What I claim as new,,a nd desire to secure by Letters Patent oi the United States, is:

l. The method of' providing an electrical coil mica tape over the entire surface -of said coil.

heating said taped coil to a temperature above 0 the softening point of the cellulose ester but the solvent and plasticizer therefrom, shrinks insuflicient to afiect injuriously the cellulose ester,

subjecting said heated coil .to a vacuum treatment to, remove the volatile ingredients therefrom, immersing said heated coil in a heated viscous fluid medium, and applying pressure to said fluid medium effectively to force the combination tape into intimate contact with thecoil, the cellulose ester-mica tape on said coil being protected from contact with the fluid medium during pressure treatment by means of a wrapping of sacrifice tape impervious to the heated fluid medium, said tape being applied to the coil over the cellulose ester-mica tape before im'- mersion thereofin said fluid medium.

2. The method of providing an electrical collpreferably about a vacuum treatment to remove substantially all 10 of the volatile material therefrom, immersing said vacuum-treated coil in a heated fluid medium incapable of penetrating the fibrous protective layer covering the cellulose ester tape, and subjecting said fluid medium to a superatmospheric pressure of such magnitude as to consolidate and compact the cellulose ester-mica tape on said coil.

EDWARD S. DAWSON, JR. 

